Lesson 4

4

Gravity: It Pulls Us Together

How does an invisible force shape everything in the universe?

Gravity pulls objects with mass toward each other and shapes motion throughout the universe. Falling objects, the Moon's orbit, and the planets' paths around the Sun provide examples of how it works.

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Key Ideas

• Gravity is an invisible force that pulls objects with mass toward each other.
• The Sun's gravity keeps all the planets orbiting around it.
• Earth's gravity keeps the Moon near us and keeps us on the ground.
• An orbit is a balance between motion carrying the planet forward while gravity pulls inward.
• The more mass an object has, the stronger its gravitational pull.

Spines

• DK's Science as You've Never Seen it Before: pg. 102-103 (Gravity)
• Mammoth Science: Gravity pg. 116
• Alternative: NASA Kids’s Gravity (https://spaceplace.nasa.gov/what-is-gravity/en/)

Vocabulary

• Gravity: An invisible force that pulls objects with mass toward each other; the greater the mass, the stronger the pull.
• Mass: The amount of matter in an object; mass determines how strong an object's gravitational pull is.
• Orbit: The path an object follows as it continuously 'falls' around another object due to gravity.
• Weight: The force of gravity pulling on a mass; weight changes with gravity's strength, but mass stays the same.

Discussion Questions

1. What does gravity do?
   Sample Answer: Gravity pulls things together.
2. What keeps the planets going around the Sun?
   Sample Answer: The Sun's gravity keeps the planets going around it.
3. Why don't planets fall straight into the Sun?
   Sample Answer: Planets are moving forward while gravity pulls them inward, creating a balanced orbit.
4. If you traveled to a smaller planet with less mass, could you jump higher or lower? Why?
   Sample Answer: You would jump higher because there is less gravity.

Digging Deeper

1. What do you think a Black Hole is, based on what you know about gravity?
   Sample Answer: Both stars and black holes have gravity, but a black hole is like a star that has been squeezed down into an incredibly tiny, super-dense space. Because all that mass is packed so tightly, the gravity near it becomes so strong that not even light can jump high enough to get away. A regular star is big and "fluffy" by comparison, so light and heat can still escape and reach us.